1. Field of the Invention
The present invention relates to a pixel structure of a display and a driving method thereof, and more particularly to a pixel structure of a display and a driving method thereof, which compensate threshold voltages of the transistors thereof.
2. Description of the Related Art
Array displays include liquid crystal displays (LCD), inorganic and organic light emitting diode (LED) displays, etc. As to LCD, backlight modules, liquid crystal and thin film transistors in pixels are used to generate images. During displaying, the backlight modules should continuously generate light for the electronic devices, such as notebooks or PDA. The operation of the displays thereof will consume substantial power. Contrary, organic LED displays uses pixels on demand for displaying and consuming less power.
Moreover, organic LED displays also have the other advantages, such as high luminance, low power consumption, wide viewing angles, low costs, and low weight. Therefore, organic LED displays gradually have been applied to different display applications. Referring to FIG. 1, a pixel structure of the active-matrix-addressed organic LED display includes two N-type thin film transistors 110 and 120. A row selecting line 110a is adapted to turn on the thin film transistor 110, in order to apply the voltage of the data signal line 110b to the capacitor 140 for driving the thin film transistor 120 as to generate light.
Although the active-matrix-addressed organic LED displays have the aforementioned advantages, the luminance thereof is not stable, caused by several reasons. One of them is that because the luminance of the organic LED is proportional to the current, the threshold voltage of the thin film transistor 120 shifts during a long-time operation as to cause the instability of the current flowing therethrough. Another reason is the process inconsistence of the thin film transistors within each pixel resulting in different threshold voltages. Accordingly, the light generated therefrom is not stable. In addition, the material of the organic LED is another reason causing the problem. The turn on voltage of the organic LED (OLED) will be shifted because of an operational temperature change.
James L. Sanford and Frank R. Libsch, of IBM inc., disclosed a pixel structure of LED display, titled “TFT AMOLED Pixel Circuits and Driving Methods,” in Society For Information Display (SID). Please refer to FIGS. 2A and 2B. A pixel structure of a display is shown in FIG. 2A and the pixel structure includes three N-type transistors 210, 220 and 230. A gate terminal of the transistor 210 is electrically connected to a row selecting line 210a, a source terminal thereof is electrically connected to a data signal line 210b, i.e. a data signal line and a drain terminal thereof is electrically connected to the transistors 220 and 230, and to a light emitting diode 240 via a capacitor 250. A gate terminal of the transistor 220 is electrically connected to an autozero line (AZ). The capacitor 250 is disposed between the gate and source terminals of the transistor 230 for storing the threshold voltage and the data voltage. FIG. 2B is a timing diagram of the pixel structure of the display shown in FIG. 2A.
The driving time of the organic LED display includes three time zones. The first time zone is used to store the threshold voltage in the capacitor 250. The second time zone is used to write in data. The third time zone is used to display. The step of writing in the threshold voltage includes: maintaining the AZ signal in a high state, Vca, for storing the threshold voltage in the capacitor 250; raising the Vca to 10 V for turning on the thin film transistor 230; and lowering the Vca to 0 V for charging the capacitor 250 to the threshold voltage of the thin film transistor 230.
Then, the Vca is 0 V and the AZ signal is in a low state so that the data is written in. If the voltage drop on the light emitting diode 240 does not change, the voltage of the capacitor 250 will be Vdata+Vt, where the Vdata means the voltage for the data and the Vt means the threshold voltage. After the data is written in, the Vca is −18 V. A current flowing through the thin film transistor 230 is proportional to (Vdata+Vt−Vt)2, i.e. (Vdata)2.
FIG. 2C is a drawing showing luminance with the data voltage Vdata for the modified voltage follower (solid) and a standard voltage follower (dashed) circuits. The line (A) represents the pixel structure of FIG. 2A; the dash line (B) represents the conventional pixel structure of FIG. 1. Under the same operation of Vdata, the former has a better luminance than that of the later. FIG. 2D a drawing showing luminance difference with the data voltage Vdata for the modified voltage follower (solid) and a standard voltage follower (dashed) circuits, when the variations of the data voltage Vdata and the threshold voltage are under 2 V. The line (C) represents the pixel structure of FIG. 2A; the dash line (D) represents the prior art pixel structure of FIG. 1. When Vdata is higher than 2.5 V, the former has a worse luminance than that of the later by 20%. If Vdata is less than 2.5 V, it will be much worse. The reason of the issue is that the thin film transistor 230 induces the voltage of the light emitting diode 240 to 0 V during the writing of the data. In addition, different threshold voltages are applied to the capacitor 250 when Vca is introduced from Vt to −18 V. Therefore, the issue will affect the operation of the organic LED display.